1. Field of the Invention
This invention relates to UV absorbing glass useful for lamp envelopes. More particularly, this invention relates to a soda-lime glass which absorbs UV radiation below 320 nm and which contains both iron oxide and cerium oxide as the UV absorbing materials in the glass.
2. Background of the Disclosure
Electric arc discharge lamps having mercury in the arc discharge emit ultraviolet (hereinafter "UV") radiation which at wavelengths below 320 nm are harmful to the human eye, fabrics, plastics and other materials. Such lamps include fluorescent lamps, mercury vapor lamps and metal halide lamps. For high intensity arc discharge lamps such as the mercury vapor and metal halide lamps, UV emission has been substantially reduced or eliminated by the use of such lamps in fixtures having lenses which absorb UV radiation and also by using outer glass jackets surrounding the lamp wherein the jacket contains UV absorbing materials. With fluorescent lamps, the combination of one or more phosphor layers adjacent the inner wall of the lamp envelope and a lamp envelope containing relatively large amounts of iron oxide in the glass has been sufficient to reduce the UV radiation emitted by these lamps to satisfactory levels that are not harmful to the surrounding environment or to humans. Iron oxide absorbs UV radiation, including the harmful UVB radiation between 280-320 nm. However, the use of significant quantities of iron in lamp glass tends to make the glass slightly discolored, thereby reducing the visible light emitted by the lamp and effecting the color of the light.
UV absorbing glass for various uses such as lenses, insulators and lamp glass envelopes are known and have had incorporated therein various amounts of UV absorbing material such as oxides of titanium, cerium, iron, vanadium, manganese and the like. Many of these materials result in a green or brown tinted glass as is disclosed in U.S. Pat. No. 2,582,453. Combinations of cerium oxide along with vanadium oxide and titanium dioxide have also been disclosed in U.S. Pat. No. 2,862,131 and U.S. Pat. No. 3,148,300 as useful for UV absorbing soda lime glasses used for fluorescent lamp envelopes. However, the use of vanadium as an ingredient in glass presents its own problems in that vanadium oxide produces a glass with a greenish yellow or amber color, which glass has been suggested for tinted ophthalmic lenses (U.S. Pat. No. 2,582,453). Vanadium is also known to volatilize, thereby contaminating the surrounding atmosphere during the glass manufacturing process and it also combines with the surface of hot fire brick to form a low melting slag, thereby corroding the fire brick and reducing furnace life. Although cerium absorbs UV radiation, it is a comparatively expensive material and its use can significantly increase lamp cost, particularly when used in the quantities disclosed in the prior art. Consequently there is still a need for yet further improvements to UV absorbing soda-lime glass useful for lamp envelopes which will transmit the visible light emitted by the light source (the visible light region of the electromagnetic spectrum is about 400-720 nm) and at the same time eliminate or substantially reduce the UVB radiation emitted by the arc discharge having a wavelength between 280-320 nm. It would be particularly advantageous if the use of such a glass composition would not significantly increase the cost of manufacturing the lamp.